Binaural type of hearing aid that has been gaining popularity in recent years is mounted on both ears, and various modes are synchronized between the hearing aids through wireless communication.
However, it is possible that these hearing aids will end up in a state in which they cannot communicate with each other due to malfunction, communication interference, or some other such problem. Therefore, techniques have been disclosed with which there is no cooperation between the two ears if this happens, and the hearing aid device operates at just one ear (see Patent Literature 1, for example).